Digital-to-analog conversion is the process of converting digital (binary) codes into a continuous range of analog signal levels. Digital codes can be converted into analog voltage, analog current, or analog charge signals using a digital-to-analog converter (DAC). Some wireless applications require conversion of digital codes into an analog signal modulating a radio frequency (RF) carrier. In some systems, the analog output of a DAC is low-pass filtered to remove aliased components in the second and third Nyquist zones. The analog signal is then translated to a carrier frequency using a mixer. The output of the mixer can be filtered to remove one of the side bands. A power amplifier amplifies the remaining side band for transmission.
One type of DAC includes a plurality of current steering cells, where each current steering cell includes a number of switches coupled to a current source. One current steering cell architecture includes four switches so that there is constant switching activity on the drain of the current source regardless of the digital input code, resulting in code independent dynamic performance. However, such additional switches come at the expense of increased area and power.